Yeah, #1 is a toughie as you really aren't getting feedback from the machine. Is it giving you any beeps? Does it get to the "dong" sound? Worst case scenario you may need to take it to Apple and have them look at it.

For #s 2,3 and 4 you have a fighting chance, but it's vital you posses an install disk, either the one the computer came with or a generic one appropriate for your hardware (tiger forG5 / Leopard or SL for Intel). For each machine, put your install disk in immediately after turning it on while holding down the "alt" key with your other hand. This can take a while but keep the alt button down. If your computer is mostly working, you should see a grey screen with icons representing all the bootable devices the computer can find. If you don't see the hard drive here you have a good clue. You should be able to choose the install disk and boot into that. Once you're booted, don't go into the OS install. Click "tools" at the top and start going through the options, being sure to run Disk Utility and do a full checkout on the OS disk.

If you get any further actionable info. feel free to post it back here.